Each of the body's cells must maintain its acid-base balance or, more specifically, its hydrogen ion or proton concentration. Only slight changes in hydrogen ion concentration cause marked alterations in the rates of chemical reactions in the cells--some being depressed and others accelerated. In very broad and general terms, when a person has a high concentration of hydrogen ions (acidosis), that person is likely to die in a coma, and when a person has a low concentration of hydrogen ions (alkalosis), he or she may die of tetany or convulsions. In between these extremes is a tremendous range of diseases and conditions that depend on the cells involved and level of hydrogen ion concentration experienced. Thus, the regulation of hydrogen ion concentration is one of the most important aspects of homeostasis.
A shorthand method of expressing hydrogen ion concentration is pH: pH=log 1/(H.sup.+ concentration)=-log (H.sup.+ concentration). The normal cell pH is 7.4, but a person can only live a few hours with a pH of less than 7.0 or more than 7.7. Thus, the maintenance of pH is critical for survival.
There are several mechanisms of maintaining pH balance. For example, during quiescence and constitutive growth, cells appear to utilize the chloride/bicarbonate exchanger, a well-studied device which provides for proton exchange across cells such as the red cell.
In addition, during accelerated periods of growth, which are induced by mitogens, growth factors, sperm, etc., cells engage another piece of cellular equipment to handle the impending metabolic burst. This is the sodium/proton (Na.sup.+ /H.sup.+) exchanger--the "NHE," which is also called an "antiporter." Because the NHE functions in a number of roles and in a number of tissues, the body has developed a family of NHEs, and recent work has elucidated a family of NHE "isoforms" that are localized in certain tissues and associated with various functions. The NHE isoforms listed below are most likely to be significant.
NHE1 is a housekeeping exchanger and is believed to be unregulated in hypertension. It is thought to play a role in intracellular pH conduct. Also, it is believed that control of this exchanger will protect a patient from ischemic injury.
NHE1a is associated genetically with diabetes and, thus, inhibition might alter evolution of diabetes through effects on beta cells in the pancreas. In addition, vascular smooth muscle proliferation, responsive to glucose, is associated with increased expression of NHE1a.
NHE1.beta. is present on nucleated erythrocytes. It is inhibited by high concentrations of amiloride. This NHE isoform is regulated by adrenergic agents in a cAMP-dependent fashion.
NHE2 is associated with numerous cells of the GI tract and skeletal muscle. Inhibition could alter growth of hyperplastic states or hypertrophic states, such as vascular smooth muscle hypertrophy or cardiac hypertrophy. Cancers of muscle origin such as rhabdomyosarcoma and leiomyoma are reasonable therapeutic targets.
NHE3 is associated with the colon. The work described below shows it to be associated with endothelial cells. Inhibition would affect functions such as water exchange in the colon (increase bowel fluid flux, which is the basis of, e.g., constipation), colonic cancer, etc. On endothelial cells, normal growth would be inhibited through inhibition of the exchanger.
NHE4 is associated with certain cells of the kidney. It appears to play a role in cellular volume regulation. Specific inhibitors might affect kidney function, and hence provide therapeutic benefit in hypertension.
NHE5 is associated with lymphoid tissue and cells of the brain. Inhibition of NHE5 should cause inhibition of proliferative disorders involving these cells. NHE5 is a likely candidate for the proliferation of glial cells in response to HIV and other viral infections.
As indicated by the above, although the NHE functions to assist the body, the inhibition of NHE function should provide tremendous therapeutic advantages. For example, although the NHE normally operates only when intracellular pH drops below a certain level of acidity, upon growth factor stimulation the cell's NHEs are turned on even though the cell is poised at a "normal" resting pH. As a consequence, the NHEs begin to pump protons from the cell at a pH at which they would normally be inactive. The cell undergoes a progressive loss of protons, increasing its net buffering capacity or, in some cases, actually alkalinizing. In settings where the pump is prevented from operating, the growth stimulus does not result in a cellular effect. Thus, inhibitors of the NHE family are likely to exert growth-inhibitory effects.
During severe acid stress--the condition that a tissue might find itself in when deprived of oxygen (or a blood supply)--the NHE family is believed to contribute to subsequent irreversible damage. For example, when blood flow to the heart is impaired, local acidosis occurs. Heart muscle cells develop a profound internal acidity. The acidity, in turn, activates otherwise dormant NHEs. These exchangers readily eliminate protons from the cell, but in exchange for sodium. As a consequence, intracellular sodium concentrations rise. Subsequently, the sodium-calcium exchanger is activated, exchanging internal sodium for external calcium. The rise in internal Ca.sup.+ concentrations leads to cell death, decreased contractility, and arrhythmias. Thus, post ischemic myocardial damage and associated arrhythmias are believed to arise from an NHE-dependent mechanism, and inhibition of this NHE should therefore prevent such occurrences. If the NHE inhibited the internalization of Na.sup.+ and slowed down metabolic activity as a consequence of the depressed pH, damage of the cell could be avoided. Hence, there is an interest in the development of NHE inhibitors for use in cardiac ischemia.
Other members of the NHE family appear to play a more classical role in water and sodium transport across epithelial surfaces. Specifically, the NHE3 isoform found in the colon is believed to play a role in regulating the fluid content of the colonic lumen. This pump is inhibited in cases of diarrhea. The NHE3 isoform present on the proximal tubules of the kidney is believed to play a similar role with respect to renal salt and acid exchange. Accordingly, inhibitors of the NHE family have been regarded as therapeutic modalities for the treatment of hypertension.
In view of the expected value of the inhibition of NHE action, scientists have sought out NHE inhibitors. The most widely studied inhibitor of NHE is amiloride, a guanidine-modified pyrazine used clinically as a diuretic. A number of derivatives have been generated, incorporating various alkyl substitutions. These derivatives have been studied with the several isoforms of NHE that are known and described above, except for NHE5, for which there is no known inhibitor.
The activities of these inhibitors against these specific exchangers have been previously determined. As seen in Table A below, each exchanger exhibits a different spectrum of response to each inhibitor:
TABLE A ______________________________________ Amiloride DMA MPA K.sub.i (.mu.M) K.sub.i (.mu.M) K.sub.i (.mu.M) ______________________________________ NHE1 3 0.1 0.08 NHE2 3 0.7 5.0 NHE3 100 11 10 ______________________________________ Notes: DMA = dimethylamiloride; MPA = methylpropylamiloride. See Counillon et al., Molecular Pharmacology 44, 1993, 1041-1045.
The NHE inhibitors described by Counillon et al. exhibit specificity for NHE1. They therefore serve a therapeutic value in the treatment of conditions where inhibition of this isoform is beneficial. However, these inhibitors do not target the other known NHE isoforms--e.g., NHE3 is unaffected.
NHE3, as is demonstrated below, is expressed on endothelial cells, and its inhibition results in anti-angiogenic effects. The spectrum of NHE isoforms inhibited by the aminosterol compounds in accordance with the invention are different from those inhibited by the amiloride or the Counillon et al. compounds, and have different, distinct pharmacological effects.
In addition, Counillon et al. also reported that certain benzoylguanidine derivatives inhibit other NHE isoforms. In particular, (3-methylsulfonyl-4-piperidinobenzoyl) guanidine methanesulfonate exhibits particular selectivity to the NHE1 as shown in the table below.
TABLE B ______________________________________ NHE Isoform Ki (.mu.M) ______________________________________ NHE1 0.16 NHE2 5.0 NHE3 650 ______________________________________
These benzoylguanadine compounds, which are based on the chemical structure of amiloride, exhibit greatest specificity for inhibiting NHE1 while retaining considerable activity against NHE2 and NHE3. To achieve pharmacological inhibition of NHE1, the widely distributed "housekeeping" isoform, undesirable inactivation of NHE2 and NHE3 would occur.
Those in the art have therefore continued to search for NHE inhibitors that exhibit selective action against a single, specific NHE. Such inhibitors would permit more precise inhibition of a tissue by perturbing the effect of the NHE on its growth.
Thus, artisans have recognized that the development of various NHE-specific inhibitors would allow for the development of new therapies for a whole host of diseases or conditions, including: treating arrhythmias; treating and preventing cardiac infarction; treating and preventing angina pectoris and ischemic disorders of the heart; treating and preventing ischemic disorders of the peripheral and central nervous system; treating and preventing ischemic disorders of peripheral organs and limbs; treating shock; providing anti-arteriosclerotic agents; treating diabetic complications; treating cancers; treating fibrotic diseases, including fibroses of lung, liver and kidney; and treating prostatic hyperplasia. Other therapeutic targets include: treatment of viral disease, such as HIV, HPV and HSV; prevention of malignancies; prevention of diabetes (i.e., islet cell injury); prevention of vascular complications of diabetes; treatment of disorders of abnormal neovascularization, e.g., macular degeneration, rheumatoid arthritis, psoriasis, cancer, malignant hemangiomas; prevention of vascular retenosis; prevention of hypertension-associated vascular damage; immunosuppression; and treatment of collagen vascular disorders.
Inhibitors of NHEs of bacteria fungi and protozoa would also be valuable as specific antimicrobials. It is known that all living cells use an NHE of one form or another to maintain intracellular Na.sup.+ and pH homeostasis. NHEs have been cloned from numerous bacteria and fungi, and bear some sequence homology to the mammalian isoforms. Using a highly specific bacterial or fungal NHE as a target, it should be possible to develop a highly specific inhibitor of such an exchanger, one that is particularly advantageous or that lacks activity against the mammalian isoforms. Such compounds would be useful as antibiotics of a different mechanism.
Thus, there is a need in the art for specific inhibitors of NHEs. There is further a need to develop NHE inhibitors for various therapeutic uses.